1. Field of the Invention
The present invention relates to a method of making a mold used to form ceramic articles by a pressure slip casting process.
2. Description of the Prior Art
A non-pressurized slip casting process is characterized by the use of a gypsum mold and has been adopted for use together with a lathe molding process and a dry press molding process. Most ceramic articles having especially large size and complex shape have been manufactured by the non-pressurized slip casting process. However, this process has a fatal defect obstructing the improvement in productivity. In the non-pressurized slip casting process, the slip cast into the gypsum mold has its water content absorbed into the mold by the mold's capillary action so that the casting rate of the slip cannot be improved drastically. When the gypsum mold is saturated with the water, its capillary action is so weakened that the mold has to be dried for a long time after every one or two uses. In order to solve those defects, a pressure casting process has recently been developed, and a variety of molds used in that process and a method of making the molds have been proposed.
One part of the molds of the prior art is formed by filling up a reinforcing pressure-resisting container or iron box with slurry or powder (e.g., a mixture of an epoxy resin and sand) to form a porous layer.
A plurality of parts thus formed are joined to form a mold cavity, and slip is introduced under a pressure of several to 30 Kg/cm.sup.2 or more into that mold cavity. This makes it necessary to clamp the mold parts firmly so that the mold can stand the pressure of the slip applied to the molding surfaces. In the mold having the structure described above, the clamping pressure is borne by the porous layer having low strength and modulus of elasticity. This also makes it necessary to make the porous layer remarkably thick. This excessive thickness invites other defects: that the compression strain of the porous layer resulting from the pressure of the slip at the pressure casting step is increased to make it liable to crack the molding surfaces of the porous layer; and that a product is bitten by the porous layers at the return of the compression strain when it is to be removed, thus making the parting itself difficult.
On the other hand, curing of the porous layer during its formation results in large shrinkage or deformation of the mold to adversely affect the accuracy of the parting face. With the accuracy of the parting face being insufficient, there arises another defect that the slip cast into the mold under high pressure will spurt or, otherwise, will be extruded from the parting face during the casting step to leave large fins.
Passages are provided in the mold for draining the water through the porous layer at the casting step and for blowing the water and air from the molding surface of the mold through the porous layer when the product is to be parted. If the mold used in the pressure slip casting process is composed of upper and lower parts, the product cannot be parted simultaneously from the upper and lower parts. Therefore, it is a common practice to part the product from one part by supplying compressed air to the part through the passages while attracting the product to the other part by evacuating the other part through the passages and then to part the product from the other part by releasing the evacuation and supplying compressed air to the other part. The passages have to be so constructed as to allow the water and air to be injected and blown uniformly from the molding surface of the mold while the product is being removed. Otherwise, the product will possibly be difficult to be released from the mold which will partially lead to inferior quality. The mold of the prior art takes no consideration in that the thickness of the porous layer is made uniform and small all over the molding surface so that the water and air may be uniformiy injected from the molding surface.